1. Field of the Invention
The present invention relates to a servo controller, and in particular, relates to a servo controller for controlling a feed axis which converts rotational motion of a servomotor into linear motion by using a ball screw.
2. Description of the Related Art
Generally, in a feed axis using a ball screw, when a moving direction of a movable body attached to the feed axis is reversed, delays occur in the reverse motion due to backlash (or mechanical clearance) and friction. The delay due to the backlash can be corrected by applying a position compensation corresponding to an amount of the backlash to a position command vale, and the delay due to the friction can be corrected by applying an amount of velocity correction for compensating the delay due to the friction to a velocity command value. In this regard, these compensations may be varied corresponding to a reverse direction (from a forward (plus) direction to a backward (minus) direction, or vice-versa).
Factors in decreasing the precision of the feed axis using the ball screw include expansion/contraction in the axial direction of the ball screw, as well as the above backlash. In order to carry out the correction in view of this matter, a technique for utilizing torque has been proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 2000-172341 describes that “in a control system for controlling torque generated by an electric motor by carrying out feedback control or feedforward control, based on a position command value which is output from a position command generating part, the torque generated by the motor or a signal corresponding to the torque is added to a position command value after multiplied by a predetermined constant value. Therefore, by a simple process for multiplying the torque by the constant value, the position command for the electric motor may be corrected by an amount of expansion/contraction of a spring corresponding to the rigidity of a machine by means of the torque generated by the motor.”
Japanese Unexamined Patent Publication (Kokai) No. 2004-187432 describes that “corresponding to an amount of torsion or expansion/contraction of a shaft estimated by a load model, a position command value is corrected so as to reduce the amount of torsion or expansion/contraction of the shaft.”
Further, Japanese Unexamined Patent Publication (Kokai) No. 2003-58213 describes that “in first to eighth embodiments, a spring constant (rigidity) is set to a constant value K0 at load correction part 9. However, when the spring constant varies depending on position θ1 of load 1, a numeric controller may be configured wherein load correction part 9 calculates load torque correction value 27 while the spring constant is variable.”
The ball screw expands or contracts in the axial direction thereof by reactive force from a movable body, such as a nut, which is threadably engaged with the ball screw. When the positional accuracy of the movable body is required to be high, it is necessary to take the amount of expansion/contraction of the ball screw into consideration. However, since this amount of expansion/contraction varies corresponding to the distance from a servomotor to the movable body, the positional accuracy may be lowered depending on the position where the movable body is accelerated or decelerated (in particular, where the moving direction of the movable body is reversed). For example, when the position compensation value is calculated by using the position where is relatively near the servomotor as a reference position, control with high accuracy can be carried out in the vicinity of the reference position, whereas the accuracy of the control is lowered at a position where is relatively away from the servomotor. To the contrary, when the position compensation value is calculated by using the position where is relatively away from the servomotor as a reference position, control with high accuracy can be carried out in the vicinity of the reference position, whereas the accuracy of the control is lowered at a position that is relatively near the servomotor.
Japanese Unexamined Patent Publication (Kokai) No. 2000-172341 suggests that, when the position command is corrected by the amount of expansion/contraction of the spring corresponding to the rigidity of the machine, the torque multiplied by a constant is added to the position command. However, this technique is not intended to carry out the correction in view of the position of the movable body on the ball screw. Also, in Japanese Unexamined Patent Publication (Kokai) No. 2004-187432, the position value is estimated by inputting a position detection value and a velocity detection value into the load model. However, the torque is not taken into consideration when calculating the estimated position value.
Further, in Japanese Unexamined Patent Publication (Kokai) No. 2003-58213, in view of the case wherein the spring constant varies depending on the position of the load, the load torque is calculated by using the spring constant as a variable value. However, it is not described that a proper position compensation value is calculated corresponding to the position of the load and the calculated value is used for feedback control.